1. Field of the Invention
The present invention pertains to an acoustic signal transmitter for logging tools characterized by a deflectable plate member which generates a pressure pulse in a liquid filled cavity which may be coupled to the formations surrounding a wellbore. The plate member is actuated by a piezoelectric actuator member.
2. Background
Certain types of acoustic well logging tools have been developed for generating so-called shear waves and compressional waves for transmission through an earth formation from fluid filled boreholes for determining certain formation characteristics in the vicinity of the borehole. Although various types of transmitters or acoustic wave generators have been developed, a continuing problem in the art of acoustic well logging tools is the development of a transmitter which is capable of operating over a suitable frequency range and also capable of imparting sufficient energy into the formation to provide acceptable signal transmission. Since it is desirable to continuously move the logging tool through the borehole during the signal generating process, the transmitter or transducer must be capable of imparting sufficient energy through the borehole fluid to and from the formation to provide an adequate return signal to the receiving elements of the tool.
Several efforts have been made to develop tools which utilize piezoelectric elements as the signal generator or transmitter. U.S. Pat. Nos. 3,593,255 to J. E. White; 4,319,345 to J. R. Dennis; 4,649,525 to F. A. Angona et al; 4,649,526 to Winbow et al; 4,715,019 to Medlin et al; 4,718,046 to Medlin; British Patent Application 2,122,351 to Exxon Production Research Company; British Patent Application 2,124,377 to Exxon Production Research Company and European Patent Publication No. 0246773 to Mobil Oil Corporation all disclose various logging tool transmitters which utilize piezoelectric signal transmitting elements. However, one shortcoming of all of the devices disclosed in the above references is that the piezoelectric members themselves are used as the primary signal transmitting or generating member. Since piezoelectric ceramic materials, in particular, have relatively small elongation or strain characteristics they do not couple acoustic energy into wellbore fluids as well as is often desired. Relatively complex piezoelectric structures are thus utilized to provide suitable mechanical advantage such as described in U.S. Pat. Nos. 4,649,525 and 4,715,019.
Accordingly there has been a long-felt need to provide an acoustic signal transmitter, particularly adapted for the dimensional constraints of wellbores, for operation in the environment of deep or shallow wellbores and which is capable of generating sufficient mechanical advantage and displacement to transmit pulse type acoustic signals through a borehole fluid for propagation through an earth formation surrounding a wellbore wherein adequate signals are transmitted back to receiver elements associated with a logging tool and the like. Such a transmitting element should have a relatively evenly distributed mechanical advantage in order to produce an asymmetric acoustic wave or pulse within the space constraints of wellbores and tools which may be inserted therein, be reliable in operation and be mechanically uncomplicated.